1. Field of the Invention
The present invention relates to a digital demodulation apparatus used in a mobile station powered by a battery.
2. Description of the Related Art
A mobile station based on standard IS-136 or GSM requires reduction of power dissipation, which increases the serviceability of a battery, i.e., the serviceability of the mobile station. For this purpose, a digital demodulation apparatus in the mobile station is intermittently operated.
In a prior art digital demodulation apparatus, a first signal generating circuit generates a first clock signal and a first frame signal which are always in an active state, and a second signal generating circuit generates a second clock signal and a second frame signal which are intermittently in an active state. An analog-to-digital (A/D) converter converts an intermediate analog signal into a digital signal. A smoothing digital filter performs a smoothing operation upon the digital signal in accordance with an internal frame signal to generate serial data. A digital signal processing circuit performs a signal processing operation upon the serial data in synchronization with the second clock signal and the second frame signal. This will be explained later in detail.
In the above-described prior art intermittent digital demodulation apparatus, however, since the smoothing digital filter stops its operation when the second signal generating circuit is switched from an active mode to an inactive mode, a large waiting time period is generated in the digital signal processing circuit after the second signal generating circuit is recovered from the inactive mode to an active mode. This increases the power dissipation, and as a result, the serviceability of the mobile station is decreased.